face is flickering in blender and invisible in unity and i cant add a texture
You have some overlapping faces. They will never render correctly. And in unity it's even worse because it uses only single-sided faces. Follow the link given by  Duarte Farrajota Ramos, there are some tips how to get rid of them.
Also try to revise your mesh to avoid face overlapping. In your case it's simpler to model it once again. Just use loopcuts (Ctrl+R) to create points, where the new faces will begin, instead of overlapping them.

look this video.